1. Field of the Invention
The present invention relates to a printing apparatus to form an image by printing a colorant on a print medium by a dot matrix printing method and also to a dot position adjusting method for the printing apparatus.
2. Description of the Related Art
As personal computers and digital cameras have come into widespread use in recent years, a variety of printing apparatus to print information output from these devices are being developed. At the same time efforts to enhance a print speed and a print quality of these apparatus are rapidly gaining momentum. A serial printer of an ink jet system using the dot matrix printing method in particular is drawing attention as a printing apparatus capable of producing a high quality printed output at low cost and high speed. Such an ink jet printing apparatus uses, for example, a bidirectional printing method as a technology for printing at faster speed. As a technology for providing a higher print quality, for instance, a multi-pass printing method is available.
In an ink jet printing apparatus a quality image cannot be obtained unless a plurality of ink droplets land at correct positions on a print medium forming dots in a correct, dot-to-dot relationship. However, various errors inherent in the printing apparatus and errors among individual print scans performed during the bidirectional printing or multipass printing unavoidably result in variations in dot landing positions. In printing apparatus of recent years, a dot alignment processing for adjusting the dot landing positions has become a necessary technology. The dot alignment processing is a method of adjusting positions on a print medium where dots are formed.
The dot alignment processing is briefly explained here. When performing a bidirectional printing, for examples variations may occur in landing positions between a forward scan and a backward scan. To correct these variations, the printing apparatus adjusts the timings at which to eject ink droplets during the forward scan and during the backward scan. An amount of correction for the alignment varies according to the printing apparatus, a print head and an environment in which the printing apparatus is used. Hence, the printing apparatus generally has a dot position adjust value calculation mode to determine an appropriate amount of correction.
In the dot position adjust value calculation mode, a plurality of line patterns are printed in forward scans and in backward scans. At this time all the line patterns are printed at a predetermined timing during the forward scans whereas during the backward scans the individual line patterns are printed by shifting the print timing by a predetermined amount from the preceding pattern. In a conventional, commonly used dot position adjust value calculation mode, a user checks a plurality of printed line patterns and selects a line pattern that exhibits the best alignment in the dot landing positions between the forward scan and the backward scan, i.e., a line pattern with the best linearity. Then, the user enters a parameter corresponding to the selected pattern directly into the printing apparatus through key manipulations. Alternatively, the user sets the dot position adjust value in the printing apparatus through an application by operating a host computer.
In more recent years, printing apparatus have been proposed which have a dot position adjust value calculation mode that permits an automatic setting of correction values without bothering the user at all. For example, Japanese Patent Application Laid-open Nos. 11-291470 (1999) and 11-291553 (1999) disclose a technology which detects printed test patterns by an optical sensor and automatically sets an adjust value obtained.
As described above, in the dot position adjust value calculation mode, a plurality of test patterns are printed on a print medium in a predetermined layout. Thus, the print medium is required to secure an area in which to print all test patterns. As to how a plurality of test patterns is detected, when an optical sensor is used for detecting the patterns in particular, it is not desired that the patterns are printed to the ends of the print medium. That is, it is desired that all the patterns be printed with some margins left at the ends.
However, ordinary printing apparatus are designed to accept various sizes of print mediums. In executing the dot position adjust value calculation mode, a print medium of a smaller size than that required by the test patterns may happen to be put on a paper feed tray. In that case, proceeding the dot position adjust value calculation mode as is may result in not all of the required patterns being printed on the print medium or a part of the individual patterns failing to be printed. In this situation the pattern detection cannot be performed normally. Further, the print medium used here is wasted. There is another problem. Since, when a print medium is fed, ink is ejected onto an area of platen that is not covered with the print medium, the platen will be contaminated with ink. If the next printing operation is executed with the platen left contaminated, another sheet newly supplied will be smeared by the platen.